CO
2 is one of the hazardous greenhouse gases causing significant changes to the environment. The sequestering of CO
2 in a suitable geological media can be a feasible method to avoid the negative effects of CO
2 emissions into the atmosphere. A numerical model was developed regarding CO
2 sequestration in a deep saline aquifer. A compositional numerical model using CMG software (GEM) was employed to study the ability of the selected aquifer to accept and retain large quantities of CO
2 injected in a supercritical state for long periods of time (up to 200 years).

Supercritical CO
2 is a one-state fluid which exhibits both gas- and liquid-like properties. In this study, supercritical CO
2 was sequestered in three forms in a deep saline aquifer. It was assumed to be supplied in an isothermal condition during the injection and sequestration processes and we ignored porosity and permeability changes due to mineralization. Also, CO
2 adsorption was not considered in our numerical model. Gas bubble formation, dissolution of CO
2 in brine and precipitation of CO
2 with calcite mineral in aquifers have been discussed. The CO
2 gas bubble displaces the formation water with immiscible behaviour. During and after displacement, the gravitational effects cause the CO
2 to rise and accumulate under the caprock. Both vertical and horizontal permeability ratios and initial pressure conditions were the most dominating parameters affecting CO
2 saturation in the three layers, whereas the CO
2 injection rate influenced CO
2 saturation in layers two and three since CO
2 was injected from layer three at the bottom of the reservoir.

Introduction

CO
2 sequestration is the capture of, separation and long-term storage of CO
2 in underground reservoirs for environmental purposes. CO
2 is one of the hazardous greenhouse gases causing significant changes in global temperature and sea levels
(1), which could have negative consequences for people in many parts of the world. Scenarios for stabilizing atmospheric CO
2 at reasonable levels will eventually require substantial cuts in overall emissions over the next few decades
(1,2). If usage of fossil fuels is to continue at current levels while avoiding undesirable climate change, technical means need to be found to reduce the carbon dioxide emitted to the atmosphere in the production and consumption of fossil fuels
(3).

CO
2 sequestration can be regarded as one possible solution for reducing CO
2 emissions in a form where they will not reach the atmosphere.

Disposal environments for CO
2 sequestration can be divided into four different categories. Oceans, terrestrial basins, a biological environment and geologic formations are the candidates for the disposal of CO
2. Among these alternatives, geologic formations can be regarded as the best possible environment to sequester CO
2 because of the fact that the storage of CO
2 in geologic formations is a self-containing and volumetrically efficient process.

In geologic formations, CO
2 can be sequestered in porous or non-porous media. Depleted oil and gas reservoirs, aquifers and coal beds can be categorized as porous media, whereas salt caverns and lined rock caverns can be regarded as types of non-porous media.